The present invention relates to a storage device and a memory for computers of the like in which a magnetic flux retained in a superconductor or a magnetic flux generated by a supercurrent is used as a carrier of information, and more particularly to a superconductor storage device which has novel means for reading out stored information and to a memory which is constructed using such superconductor storage devices as memory cells.
A superconductor storage device using a superconductor is conventionally known among storage devices in which a magnetic flux is used as a carrier of information. In the superconductor storage device, information is stored utilizing the fact that a magnetic flux produced by a supercurrent (or persistent current) flowing in the superconductor is retained with no loss of energy. The superconductor storage device is generally classified into two (first and second) types.
In the first type of superconductor storage device, logical values of "1" and "0" are discriminately stored in accordance with the (clockwise or counterclockwise) direction of the loop of a persistent (or permanent) current circulating a superconducting ring or the presence/absence of the pertistent current loop, as described by, for example, IBM Journal of Research and Development, Vol. 24, No. 2, March 1980, pp. 113-129. In other words, the logical values are discriminately stored in accordance with the direction or presence/absence of a magnetic flux which is produced by the persistent current and passes through a hole of the superconducting ring. Read-out of information in this type of storage device is effected in accordance with whether or not a Josephson junction magnetically coupled with the superconducting ring takes a resistive state.
An example of the first type of storage device is shown in FIGS. 1A to 1E. The shown device includes a loop of superconductor composed of first and second branch paths 24 and 25 of superconductor, a word line 23 for taking a supercurrent in and out of the superconductor loop, a Josephson junction 26 provided in the course of the first branch path 24, a write-in line 27 magnetically coupled with the Josephson junction 26, and a read-out line 28 having a Josephson junction 29 magnetically coupled with the second branch path 25. The storage scheme of this superconductor storage device or cell is as follows. When write-in of a logical value "1" is desired, a current is first flown in the word line 23 (see FIG. 1B) and a proper current is then flown in the write-in line 27 to generate a magnetic field. Thereby switching the Josephson junction 26 to a resistive state (see FIG. 1C). Thereafter, the current through the write-in line 27 is cut off and the current through the word line 23 is then cut off, leaving a clockwise circulating persistent current loop in the superconductor loop (see FIG. 1D). In this case, the product of the inductance of the superconductor loop and the persistent current is integer times of a flux quantum .phi..sub.0 (=2.times.10.sup.-15 Wb). When write-in of a logical value "0" is desired, a current is flown in the write-in line 27 with no current being flown in the word line 23, so that the Josephson junction 26 is switched to a resistive state. Thereby, no persistent current loop is produced in the superconductor loop. Read-out of the stored information is performed by flowing a proper current in the word line 23 after the Josephson junction 29 has been biased by flowing a proper current in the read-out line 28. In the case where the information "1" is stored and a persistent current is flowing in the superconductor loop, a part of the current injected from the word line 23 and the persistent current are flown in a superimposed form into the second branch path 25 magnetically coupled with the Josephson junction 29 so that the Josephson junction is switched to a resistive state (see FIG. 1E). Thereby, the judgment of the stored information as being "1" is made. On the other hand, in the case where the stored information is "0" or no persistent current loop is present in the superconductor loop, only the current injected from the word line 23 is flown in the second branch path 25 so that the Josephson junction 29 is maintained in a superconducting or zero-voltage state. Thereby, the judgment of the stored information as being "0" is made.
Another example of the conventional persistent current loop type of storage device is shown in FIGS. 2A to 2E. In the shown storage circuit, logical values are stored in accordance with the direction of a persistent current flowing in a superconductor loop, that is, with a clockwise circulating persistent current representing a logical value "0" and a counterclockwise circulating persistent current representing a logical value "1". Like the device of FIG. 1A, this superconductor storage device includes a loop of superconductor composed of first and second branch paths 24 and 25 of superconductor, a word line 23 for taking a supercurrent in and out of the superconductor loop, Josephson junctions 41 and 42 respectively provided in the courses of the first and second branch paths 24 and 25, a write-in line 27 magnetically coupled with the Josephson junctions 41 and 42, and a read-out line 28 having a Josephson junction 43 magnetically coupled with the second branch path 25. The storage scheme of this superconductor storage device or cell is as follows. When write-in of a logical value "1" is to be made, a proper current is first flown in the word line 23 (see FIG. 2B) and a proper current is then flown in the write-in line 27 to switch the Josephson junction 42 to a resistive state (see FIG. 2C). Thereafter, the current through the write-in line 27 is cut off and the current through the word line 23 is then cut off, leaving a counterclockwise circulating persistent current loop in the superconductor loop (see FIG. 2D). When write-in of a logical value "0" is to be made, a similar operation is performed with the inversion of the direction of the current flown in the word line 23 upon write-in of the logical value "1", thereby leaving a clockwise circulating persistent current loop in the superconductor loop. Read-out of the stored information is performed by flowing a proper current in the word line 23 after Josephson junction 43 has been biased by flowing a proper current in the read-out line 28 from the left toward the right and shown in FIG. 2E. IN the case where the information "1" is stored (or the counterclockwise circulating persistent current loop is retained), a part of the current injected from the word line 23 and the retained persistent current are cancelled in the second branch path 25 magnetically coupled with the Josephson junction 43, thereby maintaining the Josephson junction 43 in a zero-voltage state. On the other hand, in the case where the stored information is "0", a part of the current injected from the word line 23 and the retained persistent current are flown in the same direction into the second branch path 25, thereby switching the Josephson junction 43 to a resistive state. In this manner, the stored information can be read out with the discrimination of "1" or "0".
The second type of superconductor storage device, an Abrikosov flux quantum .phi..sub.0 generated in a superconductor thin film is used as a carrier of information, as described in Appl. Phys. Lett. 39(12), 15 December 1981, pp. 992-993. The Abrikosov flux quantum can maintain the magnetic flux with no loss of energy since it is produced by a persistent current flowing around the flux quantum. Logical values of "1" and "0" are discriminately stored in accordance with the quantity, the presence/absence or the direction of the Abrikosov flux quantum. Read-out of information is effected by use of a phenomenon that the critical current value of a Josephson junction is reduced by applying a magnetic field of the Abrikosov flux quantum to the Josephson junction.
An example of the above type of superconductor storage device is shown in FIG. 3. The shown apparatus includes a thin superconductor 31 for retaining an Abrikosov flux quantum therein, another superconductor 32, a flux quantum detecting Josephson junction 33 having a lower electrode provided by a part of the superconductor 31 and an upper electrode provided by the superconductor 32, a write-in line 34 provided near one end 35 of the superconductor 31, and a superconductor 36 surrounding the superconductor 31 except the end 35 thereof and having a thickness larger than that of the superconductor 31. .phi..sub.0 notionally represents the Abrikosov flux quantum retained in the superconductor 31. In this superconductor storage device, the presence and absence of an Abrikosov flux quantum to be retained in the superconductor 31 correspond to logical values "1" and "0", respectively. Namely, if the Abrikosov flux quantum is retained in the superconductor 31, the flux quantum gives an influence on the Josephson junction 33 to reduce the maximum super-current at the Josephson junction 33. This phenomenon is utilized to have the change of the maximum supercurrent correspond to the storage state "1" or "0". Write-in of "1" is performed by flowing a current of a predetermined direction in the write-in line 34 to generate a magnetic field so that an Abrikosov flux quantum having a predetermined direction (for example, an upward direction) is internally generated in the superconductor 31 from the end 35 thereof. On the other hand, write-in of "0" is performed in either one of different ways which depends on the storage state before the write-in. Namely, if the storage state before the write-in is "1", a current of the reverse direction is flown in the write-in line 34 to internally generate an Abrikosov flux quantum of a downward direction in the superconductor 31 so that it is pair-annihilated together with the Abrikosov flux quantum of the upward direction, thereby making the number of Abrikosov flux quanta zero. If the storage state before the write-in is " 0", no current is flown in the write-in line 34 so that the state is maintained as it is, thereby substantially effecting the write-in of "0". Read-out of the stored information is performed by supplying a proper current to the Josephson junction 33. Namely, if a bias current having a value between the maximum supercurrent when an Abrikosov flux quantum is retained in the superconductor 31 and that when no Abrikosov flux quantum is retained is supplied to the Josephson junction 33, a voltage is generated across the Josephson junction 33 (or the Josephson junction 33 takes its resistive state) in the case where the Abrikosov flux quantum is retained and the zero-voltage state of the Josephson junction 33 is maintained in the case no Abrikosov flux quantum is retained.